The present invention relates generally to suppression techniques for electromagnetic interference (EMI) and radio frequency interference (RFI), and, more particularly, to a method and apparatus for suppressing such EMI and RFI using active shielding techniques.
Radio frequency communications, including microwave and optical (laser) communications, have many advantages over communications at lower frequencies. Microwave communications, for example, provide a greater diversity of transmissions per frequency, enhanced accuracy and multiplexed users. One significant disadvantage of the high frequencies associated with microwave communications, however, is the ease at which the high frequency signals propagate through small gaps in circuits, cases and shields, creating stray signals.
Thus, the suppression of stray signals in microwave communication devices and other devices that operate in the presence of microwave signals assumes greater significance. A number of techniques have been proposed or suggested for suppressing stray microwave signals. Generally, such suppression techniques attempt to return the stray signals to a common ground, or to attenuate them so that they no longer have a significant amplitude. The most common approach for suppressing EMI is to shield emitting and vulnerable components inxe2x80x9cmetal cans.xe2x80x9d See, for example, P. Nyholm et at, xe2x80x9cEMI Protection in Consumer Portable Products,xe2x80x9d Electronic Packaging and Production, 40-44 (March 1994), incorporated by reference herein. Many designs for suppressing stray signals in microwave signal environments have utilized shields comprised of metal springs, clips or mesh. In addition, metal impregnated foam and rubber gasket materials have been utilized as well.
While many of these signal suppression techniques have been effective at lower frequencies, they have not been as effective at the higher frequencies associated with microwave communications. The problems associated with these schemes for microwave communications relate to the propagation of the microwave energy. Specifically, at the frequencies associated with microwave signals, the wavelength is very short. At 2 Gigahertz (GHz), for example, the quarter wavelength is only 1.9 inches. Although the gaps of cases and shields intended to suppress the stray signals are typically on the order of 0.020 inches (0.5 millimeters), the energy can propagate at a direction that is transverse to the gap height. Since the majority of equipment designs are greater than 1.9 inches on a side, this propagation theory will hold true for those specific cases.
In addition, as the frequency increases, the energy associated with the stray signals is more likely to remain on the surface of the shielding device and not penetrate into the bulk of the shielding device. This phenomenon is often referred to as the xe2x80x9cskin effect.xe2x80x9d As a result of the skin effect, all of the stray signals may not be returned to a common ground. It has been observed that above a certain frequency, such shields actually serve to re-emit the stray microwave signals.
Many people are concerned about exposure to electromagnetic radiation, including microwave radiation. In particular, there is a perceived problem regarding potential health risks associated with cellular telephones and other wireless communication devices. Specifically, many individuals are concerned about the potential danger from electromagnetic radiation associated with signals transmitting from the device. To address these concerns, most designs for cellular telephones and other wireless communication devices now incorporate electromagnetic radiation absorbing materials and other shields between the user and the antenna to shield or protect the user from the potentially harmful radiation emitting from the device.
A need therefore exists for an improved method and apparatus for suppressing electromagnetic interference and radio frequency interference in microwave communication systems. A further need exists for a method and apparatus for suppressing such electromagnetic interference and radio frequency interference using active shielding techniques. Yet another need exists for improved method and apparatus for suppressing electromagnetic interference and radio frequency interference in cellular telephones and other wireless communication devices.
Generally, a method and apparatus are disclosed for suppressing interference in a radio frequency (RF) communication system, including microwave and optical (laser) communication systems, using active shielding techniques. According to one aspect of the invention, an opposing neutralizing signal is applied to actively neutralize stray signals in an RF communication environment. An interfering or stray signal is detected using a sensing antenna, and the opposing neutralizing signal is generated in response thereto. The opposing neutralizing signal is applied to a signal neutralizing shield to shield any desired components or devices. The opposing neutralizing signal is approximately 180 degrees out of phase, and of equal magnitude to the detected interfering signal.
According to another aspect of the invention, a phase shifter/null feedback device receives the detected stray signal, if any, and adaptively adjusts the phase and magnitude of the opposing neutralizing signal. In one embodiment, the phase shifter/null feedback device includes an integrator that generates a DC value that is proportional to the energy of the interfering signal. The DC value is applied to a voltage-controlled oscillator (VCO) using known feedback techniques to adjust the phase or magnitude, or both, of the opposing neutralizing signal and to thereby minimize the DC value.
A more complete understanding of the present invention, as well as further features and advantages of.the present invention, will be obtained by reference to the following detailed description and drawings.